JP2011034057A - Optical modulator - Google Patents

Abstract

An optical modulator that has excellent reliability and can be reduced in cost and size.
An optical modulator according to an embodiment of the present invention includes a first substrate having a branch circuit composed of an optical waveguide, a second substrate having a plurality of modulation circuits, and a silica-based waveguide. And a third substrate including a multiplexing circuit composed of a waveguide. By separating the substrate according to the characteristics required for each circuit of the optical modulator, it is possible to achieve excellent reliability, low cost, and downsizing.
[Selection] Figure 4

Description

  The present invention relates to an optical modulator. More particularly, the present invention relates to an optical modulator using an optical waveguide circuit applicable to an optical communication system.

  There is a demand for an increase in communication traffic volume via the Internet and the like. Therefore, in a wavelength division multiplexing (WDM) system, an increase in transmission speed per channel and an increase in the number of wavelengths are required. Specifically, high transmission rates such as 40 Gbit / s and 100 Gbit / s are required for transmission in the WDM system. However, if the modulation symbol rate is increased for higher speed, there is a problem that the dispersion tolerance is rapidly deteriorated and the transmission distance is reduced. In addition, since the spread of the signal spectrum is increased, there is a problem that a filter band and a channel interval in wavelength division multiplexing (WDM) transmission must be increased. Thus, there is an increasing need for multilevel technology and multiplexing technology that increases the bit rate without increasing the symbol rate.

Against this background, 40 Gbis / s or 100 Gbit / s ultra high-speed transmission is actually realized or proposed per channel. As an example of such a multilevel modulator, a typical conventional example of a DQPSK modulator is shown in FIG. 1 (see Non-Patent Document 1). FIG. 1 shows an optical modulator in which an optical waveguide is formed on a lithium niobate (LN: LiNbO ₃ ) substrate using titanium (Ti) diffusion. An optical signal input from the optical fiber 101 propagates through an optical waveguide on the substrate 110, is branched by the Y branch 102 and the Y branches 103a and 103b, and is coupled to the couplers 105a and 105b via the four arm waveguides 104a to 104d. The signals are combined by the coupler 106 and output to the optical fiber 107. In this specification, such an interferometer configuration is referred to as a nested Mach-Zehnder interferometer (MZI), an inner MZI is referred to as a child MZI, and an outer MZI is referred to as a parent MZI.

  Each of the two children MZI includes electrodes 112a and 112b between the arm waveguides (GND electrodes are not shown), and constitutes a phase shifter. That is, by applying an electric field to the electrodes 112a and 112b, an opposite phase can be given to the optical signal between the arm waveguides. Such a modulator is called a QPSK modulator, and can accommodate a capacity twice as high as the baud rate. In this example, the LN substrate 110 is an X-cut substrate. The electrodes 121a and 121b are for adjusting the DC bias of the child MZI, and the electrode 122 is for adjusting the DC bias of the parent MZI.

On the other hand, as shown in FIG. 2, a conventional example in which a modulator is configured by combining an LN substrate and a quartz lightwave circuit (PLC) mainly composed of SiO ₂ glass on a Si substrate has been reported. (Patent Document 1, Patent Document 2). In FIG. 2, the LN substrate 220 is used only for the phase shifter, and quartz-based PLCs 210 and 230 are used for the optical waveguide for routing. For this reason, the characteristics of the passive circuit with excellent PLC can be utilized while maintaining the excellent characteristics of the LN optical waveguide. For example, it is possible to reduce the size of the entire circuit or reduce the entire insertion loss.

  FIG. 3 is a perspective view of a conventional example in which a modulator is configured by combining an LN substrate and a quartz-based PLC. The modulator 300 includes an optical fiber 301 on the input side of an optical signal, a quartz PLC 302 having a two-stage Y branch, an LN substrate 303 having a plurality of phase shifters, and a two-stage coupler. It is composed of a quartz-based PLC 304 and an optical fiber 305 on the optical signal output side. These substrates can be connected by a UV adhesive after aligning the respective optical waveguides.

  We have already established mass productivity and reliability for the connection between PLC and optical fiber block, and such a connection technology between substrates is easily possible because the structure is similar to the above connection. I expect. The lightwave circuit on the PLC and LN substrate can use the ones with close mode field diameter values between the optical waveguides, and even if the shape of the LN optical waveguide is horizontally long, for example, the spot size conversion function is configured by the PLC. It has already been shown that connection can be made with low connection loss. In addition, as shown in the figure, the LN substrate and the PLC often have a structure that prevents reflection by connecting the waveguide with the end face inclined.

JP 2003-195239 A JP 2003-121806 A

H. Gnauck, G. Charlet, P. Tran, P. Winzer, C. Doerr, J. Centanni, E. Burrows, T. Kawanishi, T. Sakamoto, K. Higuma, “25.6-Tb / s C + L- Band Transmission of Polarization-Multiplexed RZ-DQPSK Signals, ”Proc. Of OFC / NFOEC2007, paper PDP19, 2007.

  However, the optical modulator of FIG. 1 has a large EO constant of the LN substrate and is excellent as a high-speed phase shifter, but has the following problems.

  (1) The circuit menu of the passive circuit other than the phase shifter is not sufficient for the optical circuit of the LN substrate compared to the PLC. For example, in the LN substrate, the Y branch can be manufactured, but it is difficult to manufacture other circuits. On the other hand, in the PLC, a 1 × 2 coupler and an MZI circuit can be manufactured, and the loss is low, and a monitor waveguide can be inserted for monitoring the light intensity.

  (2) Moreover, the characteristics of the passive circuit part of the optical circuit of the LN substrate are insufficient compared to the PLC. For example, the minimum curvature radius of the bending waveguide is usually 10 mm or more, whereas PLC is 2 mm. This increases the size of the optical circuit. Further, the propagation loss of the optical waveguide is as small as 0.01 dB / cm in the PLC, whereas it is usually as large as 0.3 dB / cm in the LN, so that the loss increases. Such a drawback is more disadvantageous in the QPSK modulator as shown in FIG. 1 or in a multi-level / high-function modulator that combines them as compared with the conventional intensity modulator composed of a single interferometer.

  In contrast, the PLC-LN hybrid integrated optical modulator of FIG. 2 can improve the above-mentioned drawbacks, but has the following problems.

  (3) The PLC and the optical circuit of the LN substrate can be stably coupled with low loss, which is sufficient for constituting one MZI. However, as shown in FIG. 2, when a plurality of MZIs are configured, the number of waveguides coupled between the substrates increases, so that it is difficult to stably couple with low loss.

  (4) In the optical modulator of the LN substrate, there is a phenomenon that the phase value with respect to a certain applied voltage slightly drifts when used for a long time. Light intensity monitoring and bias control are necessary to correct the drift. This DC bias control is performed by applying a voltage to the phase shifter of the LN substrate. Such DC bias control can also be performed by a phase shifter 231 (hereinafter referred to as TO shifter) due to the thermo-optic effect on the PLC, but since the DC bias on the LN substrate has a track record, the control is performed. It may be more convenient to use the board as is. However, it is difficult to directly use such a control board as a TO shifter, and some interface including voltage and current conversion must be newly prepared.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an optical modulator that is excellent in reliability, can be reduced in cost, and can be reduced in size.

  In order to achieve the above object, the present invention provides an optical modulator including a plurality of modulation circuits, the first modulator having a branch circuit composed of an optical waveguide. A third substrate including a substrate, a second substrate coupled to the branch circuit and including the plurality of modulation circuits, and a multiplexing circuit coupled to the plurality of modulation circuits and configured from a silica-based waveguide; And a substrate.

  According to a second aspect of the present invention, there is provided the optical modulator according to the first aspect, wherein the modulation circuit is a Mach-Zehnder optical interferometer.

  The invention according to claim 3 is the optical modulator according to claim 1 or 2, wherein the branch circuit or the combining circuit includes a Y branch, a directional coupler, a Mach-Zehnder optical interferometer, and a wavelength. It is characterized by comprising at least one of independent couplers.

  The invention according to claim 4 is the optical modulator according to any one of claims 1 to 3, wherein the first substrate and the optical waveguide on the third substrate are made of quartz glass. In the optical waveguide formed, the second substrate includes a phase shifter using an electro-optic effect.

The invention according to claim 5 is the optical modulator according to any one of claims 1 to 4, wherein the second substrate is formed of Li _1-x Nb _x O ₃ or Li _1-x Ta. characterized by comprising the _x O _3.

  According to a sixth aspect of the present invention, there is provided the optical modulator according to any one of the first to fifth aspects, wherein the optical modulator is configured as a DQ-QPSK modulator.

  The invention according to claim 7 is the optical modulator according to any one of claims 1 to 5, wherein the optical modulator is configured as an RZ-DQPSK modulator.

  The invention according to claim 8 is the optical modulator according to any one of claims 1 to 7, wherein the optical modulator includes a nested Mach-Zehnder interferometer.

  The invention according to claim 9 is the optical modulator according to claim 8, wherein a Mach-Zehnder interferometer inside the nested Mach-Zehnder interferometer is provided on the second substrate. Features.

  The invention according to claim 10 is the optical modulator according to claim 8, wherein either one of the branch circuit and the multiplexing circuit of the Mach-Zehnder interferometer inside the nested Mach-Zehnder interferometer is the optical modulator. The second substrate is provided.

It is a figure which shows the conventional optical modulator formed in the LN board | substrate. It is a figure which shows the conventional modulator comprised combining the LN board | substrate and quartz type PLC. It is a perspective view of the conventional modulator comprised combining the LN board | substrate and quartz type PLC. It is a figure which shows the structural example of the optical modulator which concerns on the 1st Embodiment of this invention. 1 is a perspective view of an optical modulator according to a first embodiment of the present invention. It is a figure which shows the cross section of the LN board | substrate connected with PLC in FIG. It is a figure which shows the cross section of the LN board | substrate connected with PLC in FIG. It is a figure which shows the other structural example of the optical modulator which concerns on the 1st Embodiment of this invention. It is a figure which shows the Y branch which can be comprised as PLC in this invention. It is a figure which shows the directional coupler which can be comprised as PLC in this invention. It is a figure which shows the Mach-Zehnder interferometer which can be comprised as PLC in this invention. It is a figure which shows the wavelength independent coupler which can be comprised as PLC in this invention. It is a figure which shows another structural example of the optical modulator which concerns on the 1st Embodiment of this invention. It is a figure which shows the structural example of the optical modulator which concerns on the 2nd Embodiment of this invention. It is a figure which shows the structural example of the optical modulator which concerns on the 3rd Embodiment of this invention. It is a figure which shows the structural example of the optical modulator which concerns on the 4th Embodiment of this invention. It is a figure which shows the structural example of the optical modulator which concerns on the 5th Embodiment of this invention. It is a figure which shows the structural example which pulls out the monitor port in this invention. It is a figure which shows another structural example which draws out the monitor port in this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Embodiment 1)
FIG. 4 is a diagram showing a configuration of the optical modulator according to the first embodiment of the present invention. The optical modulator 400 includes a Y branch 402 coupled to the input side optical fiber 401, two Y branches 403a and 403b coupled to the Y branch 402, two Y branches 403a and 403b, and an arm waveguide. Two couplers 404a and 404b to be coupled and a coupler 405 coupled to the two couplers 404a and 404b and coupled to the optical fiber 406 on the output side are provided. This optical modulator also includes a quartz PLC 410 having a Y branch 402, an LN substrate 420 having two Y branches 403a and 403b, four arm waveguides, and two couplers 404a and 404b, and a coupler 405. And a quartz-based PLC 430. The LN substrate 420 is provided with electrodes 421a and 421b that give a phase shift between the arm waveguides, electrodes 422a and 422b that adjust the DC bias of the child MZI, and an electrode 423 that adjusts the DC bias of the parent MZI. Yes.

The PLCs 410 and 430 in FIG. 4 are fabricated as quartz lightwave circuits (PLCs) mainly composed of SiO ₂ glass on a Si substrate. The PLC waveguide is an embedded optical waveguide, and the relative refractive index difference between the core and the clad is 1.5%. At this time, the minimum curvature radius is as small as 1.5 mm, and it is possible to produce a bent waveguide without excessive loss. The light propagation loss is about 0.01 dB / cm. On the other hand, the optical waveguide on the LN substrate typically has a minimum radius of curvature in the order of cm and an optical propagation loss of about 0.3 dB / cm. Therefore, the PLC is an LN optical waveguide in a passive circuit other than the phase shifter for modulation. It is better than a waveguide.

  The LN substrate 420 is an X-cut substrate, and a Ti diffusion waveguide is used as an optical waveguide. As shown in the figure, signal electrodes 421a and 421b are patterned at the center of two phase shifters (that is, between two arm waveguides), and a ground electrode (not shown) is installed on the opposite side of the optical waveguide. By applying a voltage to this signal electrode, an electric field is applied in the opposite direction to the arm waveguides on both sides, so that it operates as a push-pull type modulator. This is merely an example, and the present invention may be a dual electrode type in which an electrode is disposed directly above each phase shifter on a Z-cut LN substrate and an electric field is applied to push-pull. Alternatively, a polarization inversion type LN optical waveguide may be used, or other methods may be used.

  The characteristics of the optical modulator according to the first embodiment will be described below together with Table 1.

  Regarding the optical insertion loss, the conventional LN optical modulator has a large optical propagation loss of 0.3 dB / cm. In addition, there is a loss at the connection with the optical fiber, and the insertion loss in the circuit tends to increase. On the other hand, in the first embodiment of the present invention, a proven PLC can be used for the routing passive circuit other than the high-speed phase shifter, and the connection loss between the PLC and the LN optical waveguide is typically 0.2 dB. Therefore, as described above, the loss is low.

  Regarding the size, since the minimum radius of curvature of LN is usually several centimeters, the optical circuit becomes larger when actually laid out. On the other hand, since the PLC can be bent without excessive loss even when the minimum radius of curvature is 1.5 mm, for example, the PLC can be miniaturized.

  In FIG. 4, a child MZI is configured in the LN substrate as compared with FIG. Here, the connection portion between the PLC and the LN substrate is usually fixed by an adhesive, but there is a possibility that the characteristics may change due to a slight misalignment of the connection portion. Specifically, stress is applied to the PLC or LN substrate from the package or the like due to temperature fluctuations or changes with time. Alternatively, due to the difference in thermal expansion coefficient between the PLC Si substrate and the LN substrate, the PLC substrate rotates with respect to the LN substrate as shown in FIG. In the meantime, for example, positional deviation occurs in the two optical waveguides constituting the parent MZI, and different connection losses may occur.

  Here, the structure of FIG. 2 of a prior art example and FIG. 4 of this embodiment are compared. In FIG. 4, since the child MZI is completely closed in the LN substrate, the child MZI is not subjected to the characteristic deterioration due to the positional deviation. For this reason, the characteristic degradation of the entire nested MZI, which is a superposition of the characteristics of the child MZI and the parent MZI, is relatively small.

  Furthermore, comparing the PLC-LN connection surface C between FIG. 2 of the conventional example and FIG. 4 of the present embodiment, as shown in FIGS. 6 and 7, the present embodiment has fewer waveguide connections, The distance between the optical waveguides is small. Therefore, in the present embodiment (FIG. 4), even when the substrate rotation is shifted at the same angle, the positional shift at the optical waveguide connecting portion is reduced. As shown in FIG. 8, it is also possible to further reduce the distance between the optical waveguides in the connection portion C of FIG. 4 and reduce the influence of positional deviation due to substrate rotation.

  Considering these points, the configuration of the present embodiment (FIG. 4) is less in the connection loss of the connection portion due to the stress due to the temperature change and the time change than the conventional PLC-LN modulator (FIG. 2). It is considered that the fluctuation is relatively small and the characteristics of MZI are more stable.

  In FIG. 4, electrodes 422a, 422b, and 423 for adjusting the DC bias potential are provided. In the figure, the electrode 422a is for adjusting the DC bias of the upper child MZI, the electrode 422b is for adjusting the DC bias of the lower child MZI, and the electrode 423 is the DC bias of the parent MZI. It is for adjusting. Since these DC bias electrodes are not different from the conventional LN monolithic optical modulator (FIG. 1), the control circuit can be used as it is.

  On the other hand, in the PLC-LN type of FIG. 2, the DC bias electrodes 221a and 221b of the child MZI can be installed on the LN side, but the DC bias electrode 231 of the parent MZI needs to be configured as a TO shifter on the PLC. Although DC bias adjustment is possible, a conventional control circuit cannot be used as it is.

  Regarding the cost, since the PLC substrate uses quartz glass which is also used for optical fibers on the Si substrate, the cost per unit area is low compared to the LN substrate. Therefore, the cost of the modulator can be reduced by the amount of the small LN substrate. Moreover, in order to actually manufacture the LN optical modulator module, it is necessary to make the whole into a hermetically sealed structure. However, since the package is mainly made of metal such as Kovar or stainless steel, Material costs and processing costs can also be reduced.

  From the above, the PLC-LN optical modulator of this embodiment (FIG. 4) is superior to the conventional optical modulators of FIG. 1 and FIG.

  Note that the branch portions 402, 405, 403a, 403b, 404a, and 404b in FIG. 4 use simple Y branches in the drawing, but various optical circuits can be used depending on the material and the like. Each PLC and LN has a menu that can be created, and each has its own advantages and disadvantages. These are summarized in Table 2. Each shape is shown in FIGS. 9 shows a Y branch, FIG. 10 shows a directional coupler, and FIG. 11 shows an MZI. For the MZI, the output branching ratio of the MZI can be adjusted by changing the temperature with the TO shifter 1101 for changing the branching ratio. FIG. 12 is a so-called wavelength independence coupler. By adjusting the optical waveguide length difference ΔL between the upper and lower arms and the coupling ratios C1 and C2 of the directional couplers before and after the upper and lower arms, It is a coupler capable of reducing the wavelength dependence of each light output intensity.

  The PLC can realize all the various menus shown in the table. On the other hand, a Y branch can be produced with an LN substrate, but when it comes to a directional coupler, MZI, etc., there have been few commercial results.

  In Table 2, with respect to the monitor boat drawer, in the optical circuit other than the Y branch, as shown in FIG. 18, for example, in the multiplexing portion 1701, the 2 × 1 optical circuit is set to 2 × 2, and the monitor port is It is possible to design to draw out only 10% for monitoring. On the other hand, for the Y branch, as shown in FIG. 19, it is necessary to insert a separate tap circuit 1801 to extract the monitoring light.

As for the branching ratio adjustment, when the optical propagation loss of the PLC-LN connection part or PLC or LN varies, the branching ratio is adjusted to correct the light intensity variation between the arms or between the modulation circuits. There is a merit that it is possible.
FIG. 4 is a typical configuration example of the present embodiment, and may be configured as shown in FIG. In FIG. 13, as in FIG. 4, the DC bias control of the parent MZI is the same as in the conventional example of FIG. 1, so that the control circuit can be used as it is, and as in the conventional example of FIG. There is no need to configure the control as a TO shifter.
In FIG. 13, since the child MZI is not closed in the LN substrate, different connection loss occurs between the waveguides of the child MZI with respect to the rotational deviation at the connection portion C, and the characteristic degradation of the entire nesting method MZI is relatively Can be large. However, since FIG. 13 can reduce the area of the LN substrate compared to FIG. 4, it is advantageous for cost reduction and loss reduction.

(Embodiment 2)
FIG. 14 shows a configuration of an optical modulator according to the second embodiment of the present invention. The optical modulator 1300 includes a PLC 1310, an LN substrate 1320, and a PLC 1330, and is configured as an RZ integrated DQPSK modulator thereon. The structure of the optical signal will be described according to the flow of the optical signal. Thereafter, the optical signal is output to the optical fiber 1308 after passing through an RZ modulator for converting each signal pulse into RZ. The optical modulator 1300 includes the configuration of the first embodiment, and all the features already described in the first embodiment are similarly effective.

  As another advantage, the DQPSK modulator and the RZ modulator are connected by a folded waveguide 1306 as shown in the figure. If this waveguide is composed of an LN substrate, the radius of curvature at the time of folding is large as described above, so that the substrate size increases. If PLC is used for the folded portion as shown in FIG. 14, the curvature radius of the PLC is as small as 1.5 mm, so that the substrate size can be reduced.

  From the above, in the characteristics of Table 1, the size advantage is greater than that of the LN monolithic modulator. Further, since the length of the optical waveguide on the LN substrate is longer than that in FIG. 4, it is advantageous from the viewpoint of cost reduction due to optical insertion loss and size reduction. The superiority over the conventional example corresponding to the second embodiment is larger than the superiority over the conventional example in the first embodiment by a larger circuit scale.

(Embodiment 3)
FIG. 15 shows a configuration of an optical modulator according to the third embodiment of the present invention. Similarly to FIG. 14, the modulator 1400 includes a PLC 1410, an LN substrate 1420, and a PLC 1430, and is configured as an RZ integrated DQPSK modulator thereon. However, in this embodiment, the DQPSK modulator and the RZ modulator are all manufactured on the LN substrate. Even in such a configuration, the advantages of Table 1 such as the fact that the radius of curvature of the folded waveguide 1406 is small and the size can be reduced can be utilized, albeit partially.

(Embodiment 4)
FIG. 16 shows a configuration of an optical modulator according to the fourth embodiment of the present invention. The optical modulator 1500 includes a PLC 1510, an LN substrate 1520, and a PLC 1530, and is configured by combining two DQPSK modulators, a polarization beam coupler 1504, and a polarization multiplexing circuit 1505 thereon. . The configuration will be described in accordance with the flow of the optical signal. The input optical signal is branched by the branch circuit in the PLC 1510 via the optical fiber 1501, and each of them is incident on the DQPSK modulator having the same configuration as described in FIG. Since the structure of this modulator is the same as that of Embodiment 1, it is omitted here. The optical signals emitted from the respective modulators propagate through the waveguides 1503a and 1503b, respectively, and the optical signal propagated through the waveguide 1503b is perpendicular to the other optical signal propagated through the waveguide 1503a in the polarization converter 1504. It is converted into the polarization direction. Thereafter, both optical signals are polarization-multiplexed by a polarization beam coupler (PBC) 1505 and output to an optical fiber 1508. By using the PLC, it is possible to configure a PBC 1505 with excellent characteristics and a small insertion loss and a small polarization-dependent loss over a wide wavelength range, for example, over all the C band and L band.

  Also in this optical modulator 1500, the advantages described in Table 1 of the first embodiment are exactly the same.

  Furthermore, compared to the first embodiment, the PLC-LN type optical modulator is advantageous in comparison with the LN monolithic integrated type modulator in terms of optical insertion loss and size because of the large number of routing portions of the optical circuit. It is. Further, in terms of characteristic deterioration due to the positional deviation of the PLC-LN connection portion described in the first embodiment, the connection portion between the PLC and the LN is large, and the distance is long, so that the child is as in the present embodiment. It is considered that the merit that MZI is included in the LN circuit is great. Thus, the superiority of the fourth embodiment over the corresponding conventional example is larger than the superiority of the first embodiment over the conventional example by the circuit scale.

  Further, when a directional coupler or WINC is used for the couplers 1502a and 1502b, there is an advantage that a monitor port for monitoring the light intensity in the waveguides 1503a and 1503b can be provided.

(Embodiment 5)
FIG. 17 shows a configuration of an optical modulator according to the fifth embodiment of the present invention. FIG. 17 shows an example of a DP-QPSK optical modulator. This optical modulator 1600 includes a PLC 1610, an LN substrate 1620, and a PLC 1630, as in FIG. 16, and is combined with two DQPSK modulators, a polarization beam coupler 1604, and a polarization multiplexing circuit 1605. Configured. However, in this embodiment, the two QPSK modulators are all fabricated on the LN substrate. Even in such a form, it is possible to use a high-performance PBC 1605 as in FIG. 16, and the advantages of Table 1 are partially utilized, such as a small bending radius of the PLC.

While the present invention has been described with respect to several specific embodiments, the embodiments described herein are merely illustrative in view of the many possible embodiments to which the principles of the present invention can be applied. It is not intended to limit the scope of the invention. For example, in the above embodiment, an LN substrate (Li _1-x Nb _x O ₃ ) substrate has been described as an example, but a substrate made of an electro-optic crystal such as LT (Li _1-x Ta _x O ₃ ) may be used. . As described above, the configuration and details of the embodiment exemplified here can be changed without departing from the gist of the present invention. Further, the illustrative components and procedures may be changed, supplemented, or changed in order without departing from the spirit of the invention.

100 DQPSK modulator 101 Optical fiber 102 Y branch 103a, 103b Y branch 104a, 104b, 104c, 104d Arm waveguide 105a, 105b Coupler 106 Coupler 107 Optical fiber 110 Substrate 112a, 112b Electrode 121a, 121b Electrode 122 Electrode 200 Modulator 210 230 PLC
220 LN substrate 221a, 221b Electrode 231 TO shifter 300 Optical modulator 301 Optical fiber 302 PLC
303 LN substrate 304 PLC
305 optical fiber 400 optical modulator 401 optical fiber 402 Y branch 403a, 403b Y branch 404a, 404b coupler 405 coupler 406 optical fiber 421a, 421b electrode 422a, 422b electrode 423 electrode 1101 TO shifter 1300 optical modulator 1301 optical fiber 1306 folding back Waveguide 1308 Optical fiber 1310 PLC
1320 LN substrate 1330 PLC
1400 Optical modulator 1401 Optical fiber 1406 Folded waveguide 1408 Optical fiber 1410 PLC
1420 LN substrate 1430 PLC
1500 Optical modulator 1501 Optical fiber 1502a, 1502b Coupler 1503a, 1503b Waveguide 1504 Polarization beam coupler 1505 Polarization multiplexing circuit 1508 Optical fiber 1510 PLC
1520 LN substrate 1530 PLC
1600 Optical modulator 1604 Polarization beam coupler 1605 Polarization multiplexing circuit 1608 Optical fiber 1610 PLC
1620 LN substrate 1630 PLC
1701 Multiplexing part 1801 Tap circuit

Claims (10)

An optical modulator including a plurality of modulation circuits,
A first substrate having a branch circuit composed of an optical waveguide;
A second substrate coupled to the branch circuit and comprising the plurality of modulation circuits;
An optical modulator comprising: a third substrate coupled with the plurality of modulation circuits and provided with a multiplexing circuit composed of a silica-based waveguide.   The optical modulator according to claim 1, wherein the modulation circuit is a Mach-Zehnder optical interferometer.   3. The optical modulator according to claim 1, wherein the branch circuit or the multiplexing circuit includes at least one of a Y branch, a directional coupler, a Mach-Zehnder optical interferometer, and a wavelength-independent coupler. .   The first substrate and the optical waveguide on the third substrate are optical waveguides formed of silica glass, and the second substrate includes a phase shifter using an electro-optic effect. An optical modulator according to any one of claims 1 to 3. 5. The optical modulator according to claim 1, wherein the second substrate is made of Li _1-x Nb _x O ₃ or Li _1-x Ta _x O ₃ .   The optical modulator according to claim 1, wherein the optical modulator is configured as a DQ-QPSK modulator.   6. The optical modulator according to claim 1, wherein the optical modulator is configured as an RZ-DQPSK modulator.   The optical modulator according to claim 1, wherein the optical modulator includes a nested Mach-Zehnder interferometer.   The optical modulator according to claim 8, wherein a Mach-Zehnder interferometer inside the nested Mach-Zehnder interferometer is provided on the second substrate.   9. The optical modulator according to claim 8, wherein one of a branch circuit and a multiplexing circuit of the Mach-Zehnder interferometer inside the nested Mach-Zehnder interferometer is provided on the second substrate.

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